An early type of tuner known as a "tuned radio frequency tuner" (TRF) included several radio frequency (RF) amplifiers which were all tuned to the frequency of the RF signal of desired transmission channel followed directly by a detection section, without an intervening mixer employed in later tuners. Such a tuner could provide relatively distortion free performance due to the absence of a mixer. However, TRF tuners tended to be large in size and subject to stability and gain control problems due to the number RF amplifiers which were needed. Moreover, TRF tuners did not provide a consistent or adequate degree of signal selectivity.
The type of tuner which is primarily used today is known as a "heterodyne" or "superheterodyne" tuner. In its simplest form, known as a "single-conversion" tuner, it comprises a tunable RF amplifier followed by a frequency conversion stage, including a mixer and a local oscillator. The frequency conversion stage produces an intermediate frequency (IF) signal which corresponds to the received RF signal but has a lower frequency. The IF signal is filtered by an IF filter section and the resultant signal is coupled to a detection section. The combination of conversion stage and the following IF filter section provides a significantly better selectivity characteristic than a TRF tuner. The frequency of the local oscillator signal is offset (usually higher) from the frequency of the desired RF signal by the desired frequency of the IF signal. In a television receiver, the local oscillator signal is controlled so that it places the frequency of the IF picture carrier corresponding to the RF picture carrier at a nominal frequency, for example, at 45.75 MHz in the United States and 38.9 MHz in Europe.
A single conversion tuner can be made quite small and relatively inexpensive. However, it produces unwanted intermodulation and cross-modulation products due to third and higher order components of the signal transfer characteristics of the mixer included in the frequency conversion stage. Various unwanted conversion products, known in the tuner fields as "image", "one-half IF" and "IF beats", continue to be a problem. The IF filter is designed to minimize unwanted conversion products and also to provide rejection of responses due to adjacent channels (selectivity). Thus, the selection of the IF frequency is a compromise. As a result the rejection of unwanted conversion products and selectivity of the tuner may not be adequate.
The deficiencies of a single conversion tuner have become especially troublesome due to the increasing number of "contiguous" channels now available in large cable television systems. With the advent of digital television transmission systems, such as for high definition television (HDTV), the problem becomes still more difficult because these systems make full use of the available channel spectrum and only a small guard band of a few hundred kilohertz (KHz) exists between channels. In addition, the overall frequency response of a tuner for tuning digital television signals must be flat to the edges of the channel, but nevertheless, have a very steep "roll-off" (attenuation) at the edges for adequate adjacent channel rejection. This makes the design of an appropriate IF filter more complicated since no Nyquist slopes and sound traps, which tend to ease IF filter design, can be used in digital systems. In addition, it is contemplated that both analog and digital television signals will be transmitted during a transition period. In that case, even more adjacent channel selectivity will be required for good reception of the digital signals because digital television signals will be transmitted with much less power than analog television signals.
The "double-conversion" variation of the superheterodyne tuner was developed to overcome the shortcomings of the single-conversion tuner. In this type of tuner, a first conversion stage is followed by a first IF filter section, a second conversion stage, and a second IF filter stage. The first IF section has a very high frequency range, typically in the order of 620 MHz. The second IF section has a much lower frequency range, typically the same as that of the only IF filter section of a single conversion tuner. The second IF section is followed by a detection section.
The very high frequency of the first IF filter section places the RF signals corresponding to unwanted conversion responses such as the "image" response at frequencies readily rejected by tunable RF stages which precede the first conversion stage. The low frequency second IF provides the required adjacent channel selectivity needed for modern television reception. Unfortunately, an double-conversion tuner system requires additional RF and IF circuitry compared to a single conversion tuner, and much of the additional circuitry must function at relatively high frequencies requiring extensive shielding. As a result a double conversion tuner is relatively large in size and expensive.
Another type of tuner known as a "direct conversion" tuner has improved unwanted conversion product rejection and selectivity properties with respect the TRF and heterodyne types of tuners. A direct conversion tuner operates in accordance with a third tuning method in which the frequency of a local oscillator signal of a first frequency conversion stage is set in the middle of the frequency band of the desired channel. The product of the first conversion stage is at relatively low frequency. There are no image responses because the frequency of the first conversion stage is located with the spectrum of the desired RF signal. In addition, the very low frequency range of the signal produced at the output of the first conversion stage makes it possible to readily provide a filter which can reject adjacent channel signals.
Unfortunately, because the first local oscillator signal is centered in the frequency band of the desired channel, both the upper and lower side band of the desired channel will be converted to the frequency range of the first IF signal so that the lower side band (LSB) is in effect folded over onto the upper side band (USB) in the spectrum of the first IF signal. Since the LSB and USB occupy the same frequency range, the LSB and the USB must again be separated before detection. To accomplish this, a direct conversion tuner is arranged as is shown in FIG. 1.
Basically, the direct conversion tuner contains two channels, each with two conversion stages. The received RF signal is coupled to each of two mixers M1A and M1B via a tuned RF amplifier which provides gain and some selectivity. Desirably, the gain of the RF amplifier is automatically controlled in response to an automatic gain control (AGC) signal (not shown). The local oscillator signal generated by a first local oscillator LO1 is tuned to the center frequency too of the frequency band of the desired channel between the lower sided band (LSB) and the upper side band (USB), as is shown in FIG. 2a. The first local oscillator signal is split by a phase shifting circuit PS1 into quadrature components that are used to drive mixers M1A and M1B. The respective IF output signals of mixers M1A and M1B are filtered by two low pass filters LPF A and LPF B. Low pass filters LPF A and LPF B provide the necessary selectivity to reject the responses from the adjacent channels and higher order products of mixers M1A and M1B.
Each of the output signals of mixers M1A and M1B includes both a lower side band portion and an upper side band portion corresponding to the LSB and USB portions of the received RF signal. However, as earlier indicated, the LSB portion is folded over so that it is superimposed on the USB portion and occupies the same frequency range, as is shown in FIG. 2b. The output signal of low pass filters LPF A and LPF A are coupled to respective ones of a second pair of mixers M2A and M2B. Mixers M2A and M2B are driven by respective ones of a second pair of quadrature local oscillator signals generated by a second local oscillator LO2 and a second phase shifting circuit PS2. Each of the second local oscillator signals has a frequency .omega..sub.N located above the cutoff frequency of the low pass filters LPF A and LPF B filters to fulfill the Nyquist criteria. The output signals of mixers M2A and M2B are added in a summer unit SU to produce an output signal which has a spectrum which includes separated lower and upper side band portions, as shown if FIG. 2c. This output signal is coupled to a demodulator (not shown) which demodulates it, and the demodulated resultant is coupled to further signal processing sections.
The operation of the direct conversion tuner shown in FIG. 1 can be mathematically understood by considering a very simple case in which the received RF signal is assumed to include a sinusoidal upper side band component of sin (.omega..sub.0 +.omega..sub.1) and a sinusoidal lower side band component of sin (.omega..sub.0 -.omega..sub.2), as is indicated in FIG. 3a. It is also assumed that the gains and phase shifts of the two channels are identical. The phases of the signal components produced at various point of the direct conversion tuner are indicated by the vector arrows in FIG. 1. Further, the coefficients of the various mathematically factors corresponding to signal components have been normalized in the following description.
The quadrature first local oscillator signals applied to first mixers M1A and M1B are expressed as sin .omega..sub.0 and cos .omega..sub.0, respectively; and the quadrature second local oscillator signals applied to second mixers M2A and M2B are expressed as sin .omega..sub.N and cos .omega..sub.N, respectively. The following signal is produced at the output of low pass filter LPF A: EQU cos .omega..sub.1 +cos .omega..sub.2
The following signal is produced at the output of low pass filter LPF B: EQU sin .omega..sub.1 -sin .omega..sub.2
The spectra at the outputs of low pass filters LPF A and LPF B are shown in FIG. 3b.
The result of the second mixing operation by mixer M2A produces the following output signal: EQU sin (.omega..sub.N +.omega..sub.1)+sin (.omega..sub.N -.omega..sub.1)+sin (.omega..sub.N +.omega..sub.2)+sin (.omega..sub.N -.omega..sub.2)
The result of the second mixing operation by mixer M2B produces the following output signal: EQU sin (.omega..sub.N +.omega..sub.1)-sin (.omega..sub.N -.omega..sub.1)-sin (.omega..sub.N +.omega..sub.2)+sin (.omega..sub.N -.omega..sub.2)
The addition of the two output signals of mixers M2A and M2B by summer SU results in the following signal: EQU sin (.omega..sub.N +.omega..sub.1)+sin (.omega..sub.N -.omega..sub.2)
The spectrum at the output of summer SU is shown in FIG. 3c.
The operation of the direct conversion tuner depends on the cancellation of unwanted components developed in the two channels (compare the output signals of mixers M2A and M2B indicated above including the terms sin (.omega..sub.N -.omega..sub.1) and sin (.omega..sub.N +.omega..sub.2)). As was stated above, the description of the operation of the direct conversion tuner so far provided assumes that the gains and phase shifts of corresponding elements of the two channels are identical, resulting in perfect cancellation of the unwanted components after the addition of the output signals of the two channels by summer SU. However, in practice, gain and phase characteristics of the two channels are unequal and change with temperature and time. The gain and phase characteristics affect the phase and magnitude of the vectors shown in FIG. 1. As a result, perfect cancellation of the unwanted components no longer occurs causing the generation of unwanted spurious components in the output signal produced by summer SU and the reduction of the quality of the demodulated signal. This is especially the case when the received RF signal is relatively complex, such as a television signal, and does not simply contain a lower and an upper sinusoidal component as assumed in the above description.
The generation of unwanted spurious components when a television signal is tuned by a direct conversion tuner of the type shown in FIG. 1 is illustrated in FIGS. 4a, 4b and 4c. FIG. 4a shows the spectrum of a television signal of a single channel. It includes a picture carrier (PIX), a color subcarrier (SC) and a sound carrier (SOUND). The frequency, .omega..sub.0, of the first local oscillator signal is located approximately midway between the picture carrier and the sound carrier. FIG. 4b shows the spectrum of the signal resulting from the first mixing operation. FIG. 4c shows the spectrum of the output signal of summer SU. For each desired component of the output signal of summer SU to the right of the frequency, w.sub.N, of the second local oscillator signal, an undesired "companion" to the left exists; and for each desired component of output signal of summer SU to the left of frequency .omega..sub.N an undesired "companion" to the right exists. For instance, a "companion" of the picture carrier is present to the right of .omega..sub.N between the color subcarrier and the sound carrier. The presence of the unwanted "companions" causes annoying beat patterns in the demodulated video signal and may also adversely affect the demodulated sound signal. Such unwanted components should desirably be suppressed in the order of 45 to 50 dB for optimum performance of the television receiver. This means that the gain and phase errors should desirably be kept less than 0.05 dB. and 0.5 degrees, respectively, for optimum performance of the television receiver. Such performance standards cannot be obtained and maintained with manual adjustments.